1. Technical Field of the Invention
The present invention relates to extremely excellent, drying-resistant practical baker""s yeast. Some types of drying-resistant baker""s yeast have heretofore been produced. For their forms as commercial products, they are of so-called dry yeast, and are suitable to long-term storage. However, only two types of dry yeast are commercially available, of which (1) one is for baked goods having a relatively low sugar concentration (for example, French bread, etc.), and (2) the other is for high-sugar dough (for example, for sweetened buns, etc.) having a relatively high sugar concentration. Baked goods as produced with the dry yeast of those types are not characterized by their tastes. For its use in dough, the conventional dry yeast is selected merely depending on the sugar concentration in dough as roughly classified into two, high-sugar dough and low-sugar dough, since its ability to expand the dough is relatively high in the selected condition. The yeast of the invention is characterized not only by its strong power to ferment dough having specific sugar concentrations but also by its ability to give baked goods having characteristic and delicious tastes. Specifically, the yeast of the invention has good drying resistance while retaining the properties of practical baker""s yeast. According to the invention, therefore, there are provided many dry yeast products which are useful in backing having good storability To that effect, the drying-resistant practical baker""s yeast of the invention is extremely excellent.
Specifically, according to the invention in which is employed a novel constitution of merely inactivating only either one of the ATH1 gene or the NTH1 gene in practical baker""s yeast having various excellent characteristics but not having resistance to drying, it has become possible to make the practical baker""s yeast have drying resistance that is comparable to or higher than that of ordinary commercially-available baker""s yeast for dry yeast. Therefore, the invention makes it possible to provide dry yeast having good storability and suitable to any baking methods for producing various types of baked goods, and the invention greatly contributes to developments in the baking industry.
2. Prior Art
For its commercial products, drying-resistant baker""s yeast is produced in the form of so-called dry yeast. Commercial products of dry baker""s yeast may be classified into two, depending on their production methods and properties. One is so-called active dry yeast that does not require any special apparatus for its production. This has a water content of around 10% and is generally granular, and its life for storage is relatively short. In its use, it is dissolved in warm water containing sugar, re-activated therein for ten minutes, and mixed with dough. The other is so-called instant dry yeast. This has a water content of around 4% and is storable for a long period of time (for example, for a few years in vacuum packages). In its use, it is once dissolved in warm water for re-hydration, and immediately mixed with dough without being intentionally re-activated, or it may be directly mixed with dough without being re-hydrated.
However, as so mentioned hereinabove, the conventional dry baker""s yeast has a few variations. Given that situation, it is strongly desired in the art to develop excellent practical baker""s yeast having good drying resistance and retaining the properties of ordinary baker""s yeast (therein, the property of rapid expanding dough to give baked goods having good tastes and pleasant to the tongue).
The drying resistance as referred to herein is meant to indicate the characteristic especially indispensable to the strains to be used for producing the instant dry yeast of dry baker""s yeast mentioned above. Specifically, the drying resistance is the ability of those strains to well grow in industrial cultivation conditions of fed-batch culture and to be well dried in particular industrial drying method of fluidized-bed drying to give dry yeast. In addition, the dry yeast must be well stored long, and immediately after re-hydrated with water, it must have the ability to well expand dough.
We, the present inventors have previously succeeded in obtaining frozen dough-resistant and high-sugar dough-resistant, practical baker""s yeast by increasing its trehalose retentiveness, and have filed patent applications for it (see Japanese Patent Application No. 8-297886 for NTH1 (neutral trehalase gene) disruption; Japanese Patent Application No. 9-352016 for ATH1 (acidic trehalase gene) disruption; the report of Takano, et al. of National Food Research Institute of the Ministry of Agriculture, Forestry and Fisheries of Japan, red in the Meeting of the Japanese Society for Food Science and Technology in 1997).
On the other hand, it was believed that baker""s yeast could have drying resistance only when its trehalose content could be increased. For this, John Kim, et al. (of the California University) suggested trehalase gene disruption in baker""s yeast (see Applied and Environmental Microbiology, Vol. 62, No. 5, 1996). However, they used laboratory strains in their studies, and nothing is suggested for the usefulness of the ATH1 gene disruption in practical baker""s yeast. In other words, their studies were not confirmed in the practical baking industry, and at present, the industrial usefulness of the ATH1 gene-disrupted strain is not confirmed at all. In fact, even if its trehalose content is increased, baker""s yeast could not get drying resistance only through the increase. This is because dried cells (for dry yeast) require trehalose as the energy source of reactivation when they are re-hydrated with water. Therefore, the laboratory strains having been so genetically manipulated through NTH1 gene a disruption that their neutral trehalase activity is 0 (zero) could not get drying resistance. For the same reasons, it cannot be believed at all that the laboratory strains naturally having a lower degree of NTH1 activity than that of practical strains and having been genetically manipulated for ATH1 gene disruption could still retain the trehalase activity that is necessary for re-activation of the dried cells with water only by the action of the NTH1 gene therein.
Where the yeast of Saccharomyces cerevisiae is used as practical baker""s yeast, its source shall be a species of Saccharomyces cerevisiae, and, in addition, the yeast must satisfy various requirements needed in its practical use. For example, the indispensable requirements for the yeast are as follows: 1. In order to cultivate it on an industrial scale, the yeast must grow well in blackstrap molasses media that are generally used for mass-cultivating baker""s yeast (that is, the growth rate of the yeast is high, and the yield thereof is large), the baker""s yeast having grown in the media must be efficiently separated therefrom, and the efficiency in the dehydrating step in the process of forming the thus-separated baker""s yeast into commercial products must be high (in that step, used is a specific device of a so-called dehydrator). 2. The products could keep the yeast activity (dough-expanding activity) during long-term cold storage (generally, for a few weeks), they are hardly softened during the storage, they are white without adsorbing blackstrap molasses. Apart from those, the yeast is required to have capabilities compatible with various baking methods.
For the reasons noted above, there is almost no possibility that laboratory strains not subjected to the screening for practical baker""s yeast could be directly used as those for practical baker""s yeast. In general, the requirements noted above are rarely governed by one gene but are often defined by a plurality of genes. In addition, there are known few studies relating to the relationship between the properties of practical baker""s yeast and the genes constituting the yeast.
Being different from laboratory strains, most strains for practical baker""s yeast are polyploidal and hardly form spores. Therefore, even the application of the breeding method of mating/spore separation that is based on classic breeding systems and is generally applied to laboratory strains, to baker""s yeast strains is extremely difficult. Good practical baker""s yeast shall exclusively rely on the screening of spontaneous mutants from ordinary strains that have been practically used in the art for many years, or on the screening thereof from the natural world. Gunge, et al. first applied a classic breeding method to baker""s yeast in their rare-mating method (see Genetics, Vol. 70, 1972), and after that, one example of practical application of the method was first described in the specification of a patent application filed in 1982 for xe2x80x9cSaccharomyces species FD612xe2x80x9d (see JP-B-1-16155).
As has been so mentioned hereinabove, dry baker""s yeast has a few variations. This is because screening of strains for drying-resistant yeast requires much labor since it is not clarified as yet as to what factor in yeast cells governs the drying resistance of yeast. In fact, therefore, to obtain dry baker""s yeast for practical use, yeast cells must be cultivated by fed-batch culture to make them have a suitable degree of fermenting power and a suitable trehalose content, and thereafter the thus-cultivated cells must be subjected to a dryability test using a fluidized-bed drying device comparable to an industrial one. Few attempts have heretofore been made in the art to breed strains for dry baker""s yeast through mating.
Given that situation, we, the present inventors have specifically noted trehalose of various stress-related substances in yeast cells and have made various studies relating to the intracellular trehalose content of yeast cells. In particular, we attempted to increase the intracellular trehalose which acts as a protective substance when yeast cells are dried and which is to be the energy source when the dry baker""s yeast of the dried cells is re-hydrated with water, and have completed the invention as a result of such our studies.
Even though baker""s yeast that is hardly killed by drying could be constructed through NIH1 or ATH1 gene disruption, drying-resistant practical baker""s yeast capable of producing delicious bread could not be obtained as yet. We, the present inventors desired to modify practical baker""s yeast having excellent properties but not having resistance to drying into drying-resistant practical baker""s yeast still having its original excellent properties and additionally having drying resistance that is comparable to or higher than that of ordinary, commercially-available drying-resistant yeast. For this purpose, we analyzed in detail starting yeast strains, dry yeast from the strains and even final bread as produced by the use of the dry yeast in various experiments and, as a result, have completed the invention.
The invention relates to a set of NTH1 gene-disrupted, haploid yeasts as produced through gene manipulation of disrupting the NTH1 gene in a set of haploid yeasts of which the original hybridized diploid is practical baker""s yeast.
The invention also relates to a diploid or higher polyploid, drying-resistant, practical baker""s yeast as produced through mating with one or more NTH1 gene-disrupted, haploid yeasts produced through gene manipulation of disrupting the NTH1 gene in a haploid yeast of which the diploid is practical baker""s yeast.
The invention further relates to a set of ATH1 gene-disrupted, haploid yeasts as produced through gene manipulation of disrupting the ATH1 gene in a set of haploid yeasts of which the original hybridized diploid is practical baker""s yeast.
The invention still further relates to a diploid or higher polyploid, drying-resistant, practical baker""s yeast as produced through mating with one or more ATH1 gene-disrupted, haploid yeasts produced through gene manipulation of disrupting the ATH1 gene in a haploid yeast of which the diploid is practical baker""s yeast.